Figure 2: Photograph and SEM images of the four Nutshells (NSs): AS (a, b, c); FS (d, e, f); PS (g, h, i); WS (j, k, l)
Figure 3 (a) shows the Fourier-transform infrared (FT-IR) spectra of four NS samples confirming the presence of the functional groups. All the NSs exhibit distinct peaks at 1033 cm−1, 2929 cm−1, and 3340 cm−1 which are attributed to the C-OH, C-H, and O-H bonds preset in lignocellulose.[38] Distinctive lignin peaks are found on all the NSs at 1460 cm−1, 1503 cm−1 and 1595 cm−1.[39] In addition, the hemicellulose and cellulose are confirmed by the peaks at 1054 cm−1 which is associated to the stretching vibrations of C─O─C.
Inside the nutshell structures, cellulose exists in the structure formed by lignin, while hemicellulose is tangled with cellulose, filling the gap between cellulose and lignin.[40,41] This arrangement of these biopolymers plays a crucial role in their structural properties. Certain chemical treatments of the nutshells decrease the amount of lignin and disrupt the internal links among lignin, cellulose, and hemicellulose. That results in a greater amount of negative surface charges.[37]
The zeta (ζ) potentials were analyzed for these four NSs depicted inTable 1 . The results show a consistent range of ζ values, between -20 mV to -35 mV for these four NSs. The BET surface areas were analyzed before and after the acetone and ethanol solutions treatments. The results presented in Table 1 indicates the marginal improvement of the surface areas because of this treatment.
Table 1: BET surface area and Zeta potential comparison of four nutshells